Our long term goal is to investigate specific aspects of bioenergetics in elderly subjects. The basic investigative tool in our studies is the calorimeter, and two types are available in our laboratory; one is an indirect instrument for measuring heat production (i.e., fuel oxidation), and is based on energy equivalence of oxygen consumption (VO2), carbon dioxide production (VCO2) and urinary nitrogen. The other is a direct calorimeter that measures heat release by means of heat flow across a gradient-layer. The indirect calorimeter is simpler, and is ideally suited for our studies of energy metabolism in nursing homes or hospital wards. Investigators at the beginning of the century showed close agreement between fuel oxidized and heat released. These studies were done in a small number of young subjects, and fuel oxidation was measured by carbon-nitrogen balance. Recent studies, again in a small number of subjects, confirmed that modern indirect calorimetric methods (VO2 and VCO2) accurately predicted heat evolution over 24 hours under usual dietary conditions. However, this investigation also indicated that values for 24-hour energy expenditure during over- or underfeeding, strenuous exercise, or sleep deprivation differ when measured by the two calorimetric techniques. Confirmation or refutation of this observation is essential before proceeding to our studies of energy metabolism in the aged. We therefore aim to answer two questions: 1) Is heat produced after a meal (thermic effect of food) accurately predicted from VO2, VCO2 and urinary nitrogen? and 2) Does 24-hr indirectly measured energy expenditure agree with directly measured heat release during maintenance intake and overfeeding? Undernourished male subjects (age 65-80 years, N=10; and 20-35 years, N=10) will undergo the 16-day protocol. Following baseline evaluation, the calorigenic effect of a 500 (day 2) and 1000 kcal (day 3) meal will be measured by direct calorimetry and calculated from VO2, VCO2 (continuously measured in ventilated hood) and urinary nitrogen. Then two 6-day balance periods at maintenance and repletion levels of energy intake will follow. Feeding will be continuous over 24 hrs through a naso-enteric tube. Indirect and direct measurement of energy expenditure will be performed on different 4- to 6-hour segments of the last 4 days of each balance. A 24-hr period will be created by splicing together segments from each of the 4 balance days. The results of our studies will allow us to compare the two calorimetric methods in elderly and young men following meal ingestion and during recovery from undernutrition.